Electrical discharge devices, such as igniter plugs, particularly those intended for use in aircraft engines of the jet or internal combustion types, are subject to a number of environmental extremes, extremes of temperature being probably the severest factor to which an igniter plug and its components are subjected. If the plug is of the high-voltage type, generally about 15,000 to 25,000 volts are required to cause an electrical discharge across the spark gap, which high voltages further aggravate the hostility of the environment to which the plug is exposed. Moreover, due to difficulties in properly insulating high voltage systems, flashover problems between adjacent components, in the electrical cable and in the ignition system itself are frequently encountered.
Consequently, low voltage, i.e., from about 1,000 to 5,000 volts, ignition systems have been developed, which systems require the use of a low voltage or shunted gap-type igniter plug, which is supplied with low voltage, high energy from a capacitor discharge system. In a shunt-type plug, the gap between the center electrode and the outer shell or ground electrode is bridged by a semi-conducting ceramic material which when pulses at low voltage allows the stored energy from the capacitors to discharge to ground across the igniter plug tip. Thus the ceramic tip which comprises the bridge member must possess certain physical and electrical characteristics to function properly in the environmental extremes created in modern engines and, in particular, jet aircraft engines.
In general, shunted ceramic tips are of two varieties, a surface treated insulator and the homogenous type wherein the entire insulator is a semi-conductor. An example of the former is described in U.S. Pat. No. 2,953,704, wherein an aluminum oxide ceramic insulator is coated with a sintered mixture of cuprous oxide and ferric oxide. An example of the latter is described in U.S. Pat. No. 3,558,959, wherein a semi-conducting ceramic body is formed of bonded particles of silicon carbide.
In the continuing search for improved materials to resist heat, thermal shock, spark erosion, and in general the hostile environment of modern engines, insulators comprised essentially of beryllium oxide have been found to be superior to more commonly used aluminum oxide insulators, both with respect to improved thermal conductivity and resistance to thermal shock. However, semi-conducting coatings developed for aluminum oxide based ceramics have proven unsatisfactory when applied to beryllium oxide ceramics.